1. Field of the Invention
The present invention relates generally to an encapsulated stent comprising a stent inseparably contained between two grafts wherein the, grafts are preferably comprised of expanded polytetrafluoroethylene (ePTFE) and, in addition, to a method for making an encapsulated stent. More particularly, one preferred embodiment of the present invention relates to a radially expandable encapsulated stent for implantation within the vascular system which comprises a balloon expandable tubular stent member which is sandwiched between two tubular shaped ePTFE members. The tubular shaped members may also be comprised of unexpanded polytetrafluoroethylene (ePTFE). The method for making the encapsulated stent includes concentrically positioning the stent between the graft members, applying pressure to the assembly to fuse the graft layers to one another through the openings between the struts of the stent, and sintering the assembly.
2. Description of Related Art
The use of implantable vascular grafts comprised of PTFE are well known in the art. These types of grafts are typically used to replace or repair damaged or occluded blood vessels within the body. However, once such grafts are radially expanded within a blood vessel, they exhibit some retraction subsequent to expansion. Further, they require additional means for anchoring the graft within the blood vessel, such as sutures, clamps, or similarly functioning elements. In order to overcome the retraction disadvantage and eliminate the additional attachment means, those skilled in the art have used stents such as those presented by Palmaz in U.S. Pat. No. 4,733,665 and Gianturco in U.S. Pat. No. 4,580,568, which are herein incorporated by reference, either alone or in combination with PTFE grafts, in order to provide a non-retracting prosthesis having self-anchoring capability.
For example, the stent described by Palmaz in U.S. Pat. No. 4,733,665 is used to repair an occluded blood vessel by introducing the stent into the blood vessel via a balloon catheter, positioning the stent at the occluded site contained within the blood vessel, and expanding the stent to a diameter which is comparable to the diameter of the unoccluded blood vessel. The balloon catheter is then removed and the stent remains seated within the blood vessel due to the stent""s ability to undergo radial expansion while limiting radial retraction. Further, use of radially expandable stents in combination with a PTFE graft is disclosed in U.S. Pat. No. 5,078,726 to Kreamer. Kreamer teaches placing a pair of expandable stents within the interior ends of a prosthetic graft having a length which is sufficient to span the weakened section of a blood vessel. The stents are then expanded to an increased diameter thereby securing the graft to the blood vessel wall via a friction fit.
However, although stents and stent/graft combinations have been used to provide endovascular prostheses which are capable of maintaining their fit against blood vessel walls, other desirable features are lacking. For instance, features such as increased strength and durability of the prosthesis, as well as an inert, smooth, biocompatible blood flow surface on the luminal surface of the prosthesis and an inert, smooth biocompatible surface on the abluminal surface of the prosthesis, are considered to be advantageous characteristics for an implantable vascular graft. Some of those skilled in the art have recently addressed these desirable characteristics by producing strengthened and reinforced prostheses composed entirely of biocompatible grafts and graft layers.
For example, U.S. Pat. No. 5,048,065, issued to Weldon et al. discloses a reinforced graft assembly comprising a biologic or biosynthetic graft component having a porous surface and a biologic or biosynthetic reinforcing sleeve which is concentrically fitted over the graft component. The reinforcing sleeve includes an internal layer, an intermediate layer, and an external layer, all of which comprise biocompatible fibers. The sleeve component functions to provide compliant reinforcement to the graft component. Further, U.S. Pat. No. 5,163,951, issued to Pinchuk et al. describes a composite vascular graft having an inner component, an intermediate component, and an outer component. The inner and outer components are preferably formed of expanded PTFE while the intermediate component is formed of strands of biocompatible synthetic material having a melting point less than the material which comprises the inner and outer components.
Another reinforced vascular prosthesis having enhanced compatibility and compliance is disclosed in U.S. Pat. No. 5,354,329, issued to Whalen. The Whalen patent describes a non-pyrogenic vascular prosthesis comprising a multilaminar tubular member having an interior strata, a unitary medial strata, and an exterior strata. The medial strata forms an exclusionary boundary between the interior and exterior strata. Also, one embodiment of the prosthesis is formed entirely of silicone rubber which comprises different characteristics for the different strata contained within the graft.
The prior art also includes grafts having increased strength and durability which have been reinforced with stent-like members. For example, U.S. Pat. No. 4,731,073, issued to Robinson discloses an arterial graft prosthesis comprising a multi-layer graft having a helical reinforcement embedded within the wall of the graft. U.S. Pat. No. 4,969,896, issued to Shors describes an inner elastomeric biocompatible tube having a plurality of rib members spaced about the exterior surface of the inner tube, and a perforate flexible biocompatible wrap circumferentially disposed about, and attached to, the rib members.
Another example of a graft having reinforcing stent-like members is disclosed in U.S. Pat. No. 5,123,917, issued to Lee. The Lee patent describes an expandable intraluminal vascular graft having an inner flexible cylindrical tube, an outer flexible cylindrical tube concentrically enclosing the inner tube, and a plurality of separate scaffold members positioned between the inner and outer tubes. Further, U.S. Pat. No. 5,282,860, issued to Matsun, et al. discloses a multi-layer stent comprising an outer resin tube having at least one flap to provide an anchoring means, an inner fluorine based resin tube and a mechanical reinforcing layer positioned between the inner and outer tubes.
Another stent containing graft is described in U.S. Pat. No. 5,389,106, issued to Tower. The Tower patent discloses an impermeable expandable intravascular stent which includes a distensible frame and an impermeable deformable membrane interconnecting portions of the frame to form an impermeable exterior wall. The membrane comprises a synthetic non-latex, non-vinyl polymer while the frame is comprised of a fine platinum wire. The membrane is attached to the frame by placing the frame on a mandrel, dipping the frame and the mandrel into a polymer and organic solvent solution, withdrawing the frame and mandrel from the solution, drying the frame and mandrel, and removing the mandrel from the frame.
Although the previously described reinforced grafts disclose structures which have increased strength and durability, as well as inert, smooth inner and outer surfaces to reduce thrombogenicity, the prior art references do not disclose a device which exhibits these advantageous characteristics in addition to good resistance to radial contraction and possible self anchoring means. Accordingly, there is a need for a radially expandable reinforced vascular graft having good resistance to radial contraction and means for self anchoring which also exhibits increased strength, increased durability, and inertness with respect to its interior and exterior surfaces.
It is a principal object of the present invention to provide a reinforced vascular graft in the form of an encapsulated stent having self-anchoring means, increased strength and increased resistance to radial retraction.
It is a further object of the present invention to provide a reinforced vascular graft in the form of an encapsulated stent having inert, smooth, biocompatible interior and exterior surfaces to reduce thrombogenicity associated with the surfaces of the graft.
It is still a further object of the present invention to provide a radially expandable reinforced vascular graft in the form of an encapsulated stent that can be used with any conventional balloon catheter and which easily detaches from the balloon catheter upon deflation of the balloon without exhibiting recoil of the graft onto the balloon.
It is yet a further object of the present invention to provide a one-piece radially expandable reinforced vascular graft which comprises a stent or similarly structured support layer and dual PTFE graft layers which are inseparable both in vivo and in vitro.
It is a still further object of the present invention to provide an encapsulated stent comprising a stent member sandwiched between two PTFE graft members.
It is yet another object of the present invention to provide an encapsulated stent comprising a tubular stent circumferentially sandwiched between two tubular PTFE grafts wherein the length of the PTFE grafts are less than or equal to the length of the stent at an expanded diameter.
It is still another object of the present invention to provide an encapsulated stent comprising at least two tubular stents circumferentially sandwiched between the ends of two tubular shaped PTFE grafts wherein the length of the PTFE grafts is greater than the length of the stents and a region of the PTFE grafts is unsupported by the stents. Further, the encapsulated stent may take the form of an articulated configuration in which there are a series of unsupported PTFE graft areas disposed between stent supported PTFE graft areas.
It is another object of the present invention to provide a method for making an encapsulated stent which comprises inseparable layers of PTFE graft, stent, and PTFE graft.
Briefly, the present invention generally comprises a radially expandable reinforced vascular graft which includes a first layer of biocompatible flexible material, a second layer of biocompatible flexible material, and a support layer sandwiched between the first and second layers of biocompatible flexible material. The biocompatible flexible layers are preferably comprised of expanded PTFE, but may comprise unexpanded PTFE. The support layer preferably comprises a stent and may be made of any strong material which can undergo radial expansion but resists radial collapse such as silver, titanium, stainless steel, gold, or any suitable plastic material capable of maintaining its shape and material properties at sintering temperatures and having the necessary strength and elasticity to enable uniform expansion without collapse. A preferred embodiment of the radially expandable reinforced vascular comprises a tubular stent cover having a first biocompatible flexible tubular member, a tubular shaped support member concentrically positioned about the outer surface of the first biocompatible flexible tubular member, and a second biocompatible flexible tubular member concentrically positioned about the outer surface of the tubular shaped support member wherein the tubular members form inseparable layers. The tubular shaped support member preferably comprises a plurality of openings to enable the first and second biocompatible flexible tubular members to be fused together through the openings thereby forming flexible tubular member circumferentially positioned about the outer surface of the first biocompatible flexible tubular member, and at least one tubular support member sandwiched between an end portion of the first and second biocompatible flexible tubular members such that the inner surface of the support member is adjacent to the outer surface of the first biocompatible member and the outer surface of the support member is adjacent the inner surface of the second biocompatible member thereby encapsulating the stent. A second tubular support member may be concentrically sandwiched between the opposite ends of the first and second biocompatible flexible tubular members to form a stented graft having ends comprising stents or support members encapsulated in ePTFE and a middle ePTFE graft portion that is unsupported by stents. Additional and/or other types of structural supports may also be encapsulated between inner and outer flexible biocompatible members as described above to form varying embodiments of the reinforced vascular graft. For example, an expandable, articulated reinforced vascular graft may be formed by sandwiching a structural support assembly comprising multiple stent members spaced apart from one another between two biocompatible tubular members. The resulting expandable, articulated reinforced vascular graft is a monolithic structure which is incapable of separation or delamination. The structural support assembly appears to be embedded within the wall of the expandable, articulated reinforced vascular graft.
The biocompatible material used in all of the above-described embodiments may comprise polytetrafluoroethylene, polyamides, polyimides, silicones, fluoroethylpolypropylene (FEP), polypropylfluorinated amines (PFA), or any other fluorinated polymers.
The present invention is also directed to a process for making a radially expandable reinforced vascular graft which includes the steps of:
a) positioning a layer of pressure expandable support material having a plurality of openings over a first layer of flexible biocompatible graft material having a top and bottom surface;
b) positioning a second layer of flexible biocompatible graft material having a top surface and a bottom surface over said layer of support material such that said layer of support material is sandwiched between said first and second layers of flexible biocompatible graft material; and
c) fixing said layer of support material to said first and second layers of flexible biocompatible graft material such that all of said layers are inseparable.
The step of fixing the support layer to the biocompatible graft layers may comprise applying pressure to the layers after they are loaded onto a mandrel and then heating the resulting assembly at sintering temperatures to form a mechanical bond, or applying at least one of an adhesive, an aqueous dispersion of polytetrafluoroethylene, a polytetrafluoroethylene tape, fluoroethylpolypropylene (FEP), or tetrafluoroethylene between the flexible biocompatible graft layers and the support layer and heating the resulting assembly at a melt temperature below the sintering temperature of the biocompatible graft layers.
Further, the process for making a preferred embodiment of the radially expandable reinforced vascular graft of the present invention includes the steps of:
a) placing a first expanded unsintered tubular shaped polytetrafluoroethylene graft onto a mandrel;
b) placing a tubular shaped support structure having a plurality of openings over said first graft;
c) placing a second expanded unsintered tubular shaped polytetrafluoroethylene graft having a larger diameter than said first graft over said tubular shaped support structure; and
d) fixing said first and second tubular graft members to said tubular support structure such that all of said members are inseparable.
As in the general process for making an expandable reinforced vascular graft, the step of fixing the first and second tubular graft members to the tubular support structure may comprise applying pressure to the tubular members after they are loaded onto a mandrel followed by heating the resulting assembly to sintering temperatures for the ePTFE tubular members in order to form a mechanical bond. Alternatively, the fixing process may include the step of applying at least one of an adhesive, an aqueous dispersion of polytetrafluoroethylene, a polytetrafluoroethylene tape, FEP, or tetrafluoroethylene between the tubular ePTFE grafts and the tubular support structure and heating the resulting assembly at a melt temperature below the sintering temperature of the tubular ePTFE grafts.
These and other objects, features and advantages of the present invention will become more apparent to those skilled in the art when taken with reference to the following more detailed description of the preferred embodiments of the invention in conjunction with the accompanying drawings.